1. Field of the Invention
The present invention relates to an accessory apparatus, attached to a main apparatus such as a printer, for which the operating power is supplied by the main apparatus, and relates in particular to a method for protecting a circuit in the accessory apparatus from abnormal voltages.
2. Related Arts
For a peripheral apparatus such as a printer or a computer, an accessory apparatus such as an image reader, may be connected to the main body of the peripheral apparatus (hereinafter referred to as a main apparatus). For example, if the main apparatus is a printer, an accessory apparatus (hereinafter referred to as an optional apparatus) can be a feeder unit for supplying sheets in a plurality of sizes to the main apparatus, or a double-sided printing unit for printing both sides of a sheet.
FIGS. 14A and 14B are diagrams for explaining a method used for connecting optional apparatuses to a main apparatus. In FIG. 14A, a main apparatus and each of two optional apparatuses are connected by a cable (an arrangement hereinafter referred to as a cable connection). Connectors provided at the ends of the cable are inserted into receptacles on the main apparatus and the optional apparatus. In FIG. 14B, connectors on a main apparatus and an optional apparatus are used to make a direct connection (such an arrangement is hereinafter referred to as a connector connection). Connectors on the optional apparatuses are also used to make a connector connection. Power for optional apparatuses which are thus connected to a main apparatus is supplied by the main apparatus.
Generally, when the cable connection shown in FIG. 14A is used, an electrical connection of a main apparatus and an optional apparatus is not established merely by mounting the optional apparatus on the main apparatus; a cable must be installed to connect the units. Also, if the optional apparatus is removed from the main apparatus but the two units are still connected by the cable, the electrical connection is maintained. With the connector connection shown in FIG. 14B, however, the electrical connection between the main apparatus and the optional apparatus is established when the optional apparatus is mounted on the main apparatus. In this case, the electrical connection is lost when the optional apparatuses is removed from the main apparatus.
FIG. 15 is a diagram showing an example connector used for the above connector connection. In FIG. 15, the connectors used for the connector connection are rack panel connectors, for example, mini-drawer connectors produced by AMP Corp. The rack panel connectors in FIG. 15 have like shaped male and female housings which engage each other. It should be noted that the male and female contact terminals of the rack panel connectors also have the same shape. The rack panel connectors, which are directly attached to a main apparatus or an optional apparatus, have a floating function which enables them to absorb attachment positioning errors. These connectors may be so attached that they can be displaced a predetermined distance (e.g., about 1 mm).
When the optional apparatus connected to the main apparatus using a connector is to be detached while the power source of the main apparatus is ON, the following problem arises. When, for example, the optional apparatus is attached to the main apparatus which is in the power-ON state, and the timing for the coupling of the power supply pins (e.g., +5 V or +24 V) of the connectors is shifted from the timing for the coupling of the power GND pins, an overvoltage will be applied to various circuits provided in the optional apparatus, and deterioration or damage to the circuits will occur.
When, for example, the optional apparatus is to be attached to the main apparatus which is in the power-ON state, the power supply pins for the +24 V connectors may be coupled together before the power GND pins are engaged. Then, either a current by +24 V supplied from the main apparatus, will not flow its normal route and will flow in reverse along a motor drive circuit in the optional apparatus, or an overvoltage will be applied to a control logic circuit in the optional apparatus, for which an operating voltage of +5 V is required. Thus, the motor drive circuit or the control logic circuit in the optional apparatus may be damaged, and the optional apparatus failure may occur.
In addition, when the optional apparatus is removed from a main apparatus in the power-ON state, the power GND pins of the connectors may be disengaged before the +24 V power supply pins. Then, as in the above case, since there is no contact between the GND pins, a voltage of +24 V will be supplied to the optional apparatus, and the motor drive circuit and the control logic circuit in the optional apparatus may be damaged and cause an optional apparatus failure.
In particular, as is described above, since with the connector connection the detachment of the optional apparatus from the main apparatus is easy, it is highly probable that the operator will erroneously remove the optional apparatus from the main apparatus which is in the power-ON state.